1. Field of the Invention
This invention relates to containers, adapted for holding food, which are suitable for use in microwave oven cooking.
2. Description of the Prior Art
It is known in the art to produce various shaped articles from foamed and unfoamed thermoplastic materials such as polystyrene sheet or impact modified polystyrene sheet (i.e. high impact polystyrene sheet) whether foamed or unfoamed, by thermoforming methods. Many such articles are containers used for packaging foods.
However, many thermoplastic containers are melted by cooking foods or cracked by fats and oils in foods and cannot be used to package foods that are to be heated in a microwave oven. In particular, polystyrene, polycarbonate, polymethyl methacrylate or poly(p-methylstyrene) containers are not suitable for microwave cooking. In general, containers made from thermoplastic polymers or copolymers with a softening temperature below 212.degree. F. (100.degree. C.) are not sufficiently resistant to hot foods when heated in a microwave oven.
Microwavable thermoplastic containers are described in U.S. Pat. Nos. 4,335,181, and 4,505,961. The containers disclosed in these patents were made from polymers such as polystyrene or poly(p-methylstyrene) that are not in themselves sufficiently heat resistant to maintain their structural integrity under microwave cooking conditions. Loss of structural integrity is said to be avoided by laminating to the substrates a protective film such as polyacrylonitrile, polyethylene terephthalate or a cellulose ester such as cellulose acetate or cellulose acetate propionate. However such polymers are not readily coextrudable over vinylaromatic polymers or the resulting structures are not as easily thermoformable as are structures of vinylaromatic polymers. Thus there still exists a need in the art for microwavable thermoplastic containers that are adaptable to volume production in an economical manner.